This invention relates to an apparatus for fabricating an optical fiber preform of a modified CVD type.
An MCVD method has been heretofore used an apparatus for fabricating an optical fiber preform. This method the steps of supplying a glass raw material into a rotating reaction tube, heating the raw material by a heater such as a torch reciprocating in the axial direction of the reaction tube, and accumulating the raw material in the reaction tube to form a desired refractive index distribution.
The conventional MCVD method has the following drawbacks. When the reaction tube made of quartz tube is heated and softened by the heater, it gradually contracts due to the own surface tension, with the result that a uniform glass accumulation layer having desired thickness cannot be formed on the inner surface of the reaction tube, and the reaction tube is deformed due to the softening with the result that the longitudinal section of the reaction tube becomes irregular.
In order to solve the above described drawbacks, a method of increasing the internal pressure of the reaction tube higher than the external pressure while accumulating the glass layer in the reaction tube has been proposed. More specifically, as disclosed in Japanese Patent Publication No. 37536/1982, a method of feeding back the outer diameter value of the reaction tube to the internal pressure regulator provided at the other end of the reaction tube to regulate the internal pressure of the reaction tube has been proposed. However, according to this method, since the flow rate of the supplied glass raw material and the sequentially varying internal pressure in the reaction tube by the inner diameter of the reaction tube are controlled by indirect values such as the outer diameter of the reaction tube, they cannot be accurately controlled. Further, the outer diameter of the reaction tube is likely to vary in the longitudinal direction, and the selection of the measuring point becomes difficult.
The inventor of the present invention has proposed in Japanese Patent Application No. 243360/1983 an improvement of the abovementioned conventional method. The improved method provides a pressure measuring unit, an exhaust port, a gas pressure control chamber having a gas inlet at the other end of the reaction tube, and an exhaust gas flow regulating valve in the exhaust port, so as to control the opening and closing degree of the valve according to the value of the pressure measuring unit. This method has the advantage that the internal pressure of the reaction tube can be measured directly so that one can accurately regulate the pressure as compared with the previous conventional method. However, since the pressure is regulated only by opening and closing the regulating valve, improved method still has the drawback that its responsiveness is deteriorated.